Nitrogen in ocean waters off the Pacific coast - whether it comes from land or the deep sea - fuels the explosive growth of highly toxic algae that shore dwellers think of as red tides when the microscopic plants become profuse enough to float in mats.

Researchers at San Francisco State University'sRomberg Tiburon research center have discovered that the toxic plants are stimulated to grow abundantly when the nitrogen in nutrient-rich water wells upward naturally from the ocean's depths, or when the nitrogen in sewage and agricultural runoff enters the ocean.

So-called red tides can occur any time of year. But there's no such thing as a red tide - the mats of floating algae called phytoplankton are far more often brown or purplish than red, and they are never tides.

In two years of painstaking lab experiments using chemically regulated artificial seawater laced with quantities of nitrogen-containing chemicals, William P. Cochlan of San Francisco State University and Maureen Auro, his former graduate student, studied the precise effects of nitrogen on the toxic species of phytoplankton called pseudo-nitzschia that is common in the massive blooms of algae seen in the ocean off Bay Area shores.

Cochlan and Auro also found that the growth of the hazardous algae is stimulated more strongly by natural nitrate in upwelling sea water than by land-based pollution from sewage and fertilizers containing the nitrogen compounds urea and ammonia.

The plants produce a deadly toxin called domoic acid that concentrates in crabs, mussels and other shellfish and is a powerful killer of marine mammals like sea lions, seals and sea otters.

Domoic acid can also be fatal to humans who eat the contaminated shellfish. It causes a disorder called amnesic shellfish poisoning, with symptoms that include nausea, vomiting and, in severe cases, seizures, coma and ultimately death.

Fish accumulate domoic acid in their bodies, too, but are immune to the toxin. However, it is fatal to seabirds that may feed on contaminated small fish like anchovies and sardines during an algal bloom, scientists say.

"Regardless of its source, nitrogen has a powerful impact on the growth of phytoplankton cells that are the foundation of the marine food web," Cochlan said.